Why did God put Barack Obama on this earth? If you had asked that question between his 2004 convention address and his 2008 campaign, the answer could have been summarized in a short elevator speech.


Obama was here to heal our politics, to move us beyond the stale debates and the childish partisanship that lead to stagnation, futility and silliness.


This purpose did not survive contact with reality. But President Obama found other clear missions. In 2009, his mission was to avert the worst of the financial crisis. In 2010, it was to expand health insurance coverage.


During this time, you knew what Barack Obama was about, where his priorities lay. But, since 2010, that has not been the case. Since then, Representative Paul Ryan has been driving the Washington policy debate with his plan to cut spending and restructure entitlements. President Obama, meanwhile, has produced a string of budgets so inconsequential that members of his own party have not even noticed them.


Obama has been reactive. He has been defined by the various negotiating positions he has taken in his confrontations with Congress. He’s used a more partisan political style to mask his small-bore policy substance. It’s not clear what he is passionate to do if he is elected for another four years.


The Democratic convention is his best chance to offer an elevator speech, to define America’s most pressing challenge and how he plans to address it.


He has three clear options.


First, global warming. President Obama has occasionally said he’d like to do something about climate change if he gets a second term. Given the country’s immediate economic and fiscal problems, this seems obtuse to me. But if this is really where Obama’s passion lies, he should go for it.


He could vow to double down on green energy and green technology. He could revive cap-and-trade legislation that would create incentives for clean innovation. He could propose a tax reform package that would substitute gasoline and energy consumption taxes for a piece of our current income taxes. He could say that his No. 1 international priority will be to get a global warming treaty ratified by all the major nations.


This would be a big, intellectually serious agenda, designed to address a big problem.


Second, broken capitalism. Obama could go before the convention and say that there has been a giant failure at the heart of modern capitalism. Even in good times, the wealth that modern capitalism generates is not being shared equitably. Workers are not seeing the benefits of their own productivity gains.


Obama could offer policies broad enough to address this monumental problem. He could vow to strengthen unions. He could promise to use federal funds to pay for 500,000 more teachers and two million more infrastructure jobs. He could cap the mortgage interest deduction, cap social security benefits, raise taxes on the rich, raise taxes on capital gains and embrace other measures to redistribute money from those who are prospering to those who are not. He could crack down on outsourcing and regulate trade. He could throw himself behind a new industrial policy to create manufacturing jobs.


This agenda wouldn’t appeal to moderates, or people like me, but it’s huge, it’s serious and it would highlight a real problem.


Third, Bowles-Simpson. Everybody pays lip service to the Bowles-Simpson plan to reduce the deficit, but neither campaign really embraces it.


Obama could use his convention to throw himself wholeheartedly behind the general Bowles-Simpson approach. He could argue that America is weighed down by rotting institutions and faces fiscal ruin. He could vow to push through tax reform that would lower rates and reduce loopholes. He could endorse a 22 percent cap on government spending. He could commit to limiting the growth of domestic and defense spending. He could double down on his health care cost containment ideas. He could restructure Social Security and make it more progressive.


This, too, is a big, serious agenda, addressing a real national need. This, too, is an agenda commensurate with the size of the problem that confronts us.


Personally, I wish Obama would use this convention to embrace Bowles-Simpson. That would lay the foundation for decades of prosperity. It would galvanize a new center-left majority.


But, mostly, I wish he’d be for something. I wish he’d rise above the petty tactical considerations that have shrunk him over the past two years. I wish he’d finally define what he stands for. A liberal populist? A Clintonian moderate? At some point, you have to choose.


Four years ago, Obama said we could no longer postpone tackling the big problems. But now he seems driven by a fear of defeat. His proposals seem bite-size. If Obama can’t tell us the big policy thing he wants to do, he doesn’t deserve a second term.